It's almost certainly in your user folder.
C:\User\MrTrent\Skyrim\Screenshots or C:\User\MrTrent\AppData\Roaming\Valve\Steam\Screenshots or something
You can go find the actual location yourself though.
This is Marcus, the gayest orc in Skyrim.
EDIT: You may notice i named the files the wrong way round. If you do notice, you don't need to point it out 'cos i already know.
Heh. Same here, and I picked a similar spot to take a screenshot from. And then you beat me to posting it.
In fact, I'm going to post it anyway. Let me find something to reduce the 5MB screenshot to something a bit more polite.